FIL had an old gas tank that he hauled water to livestock in for years after digging it out of the ground.  I needed a burn barrel, and since it had some pinhole leaks in it, I cut it in half and used both halves for burn barrels.  The last half is just now burning itself out, it lasted for a long time and made a good burn barrel.

 But, I swore I would never again use the torch on an old gas barrel, no matter how much it had been cleaned out.  I actually had no problems, but kept getting small pops that scared me over and over.  Evidently some of the gas or something incorporated itself into the rust, as even thought it had been used to haul water for years, then sat empty with all holes open for many years, there was enough popping that I wondered if it would explode.

This was in my younger days, and many people were doing such foolish things.  I just decided I didn't want to do them again.

People will tell you over and over that it is safe after doing some sort of procedure, but to me it just is not worth the risk.